1. Field of the Invention
The invention relates to a small size transmission adapted for use with a motor, comprising a basic transmission which has a basic transmission input shaft, borne in a housing by means of rolling bearings and at least one subsequent transmission stage which is arranged in the housing and comprises a variance gear adapted for meshing with a drive pinion of the motor and pressed onto the basic transmission input shaft from the input side of the transmission. Moreover, the invention relates to a transmission series comprising such a small size transmission, as well as to a process for assembling such a small size transmission.
2. Description of Related Art
From EP 0452739B1, a transmission series is known in which a plurality of transmissions of the same size, but with different transmission ratios, are produced by combining a so-called variance stage of the transmission with an identical basic transmission. The variation of the transmission ratios is realized by different pairs of pinions and gears in the variance stage, whereas the basic transmission, consisting of further transmission stages, stays unaltered. According to this previous publication, it is possible to minimize the number of constructive parts necessary for the production of the variance stage by using an appropriate correlation of the constructive sizes and nominal transmission ratios of the variance stage. This makes it possible to provide a large number of transmission ratios of plural-stage transmissions by combining, for each constructive size, an identical basic transmission (or some few basic transmissions) with a large number of variance stages.
On this basis it is an object of the invention to further minimize the production expenses for a transmission of the kind described.
To this end, the invention provides a support element in the housing, in fixed position relative to the housing, in such a manner that the basic transmission input shaft contacts, under the effect of the pressure force necessary for pressing the variance gear onto the basic transmission input shaft, the support, whereby the support essentially bears the pressure force and distributes it over the housing.
In the operating mode, when the small size transmission is connected to a corresponding motor (or, more generally, a drive) the variance gear meshes with the drive pinion of the motor. The drive pinion is commonly located on the motor shaft (drive shaft). Together with the variance gear, it forms the first stage of the transmission, which is a variance stage according to EP 0452739B1. Although, consequently, the pinion is a functional component of the transmission, in practice, it is constructively connected to the drive rotor. However, the invention also refers to transmission embodiments in which the pinion of the variance stage is fixed to a shaft which is a constructive component of the transmission, and which is coupled to the motor (e.g. by a flange).
The basic transmission comprises the transmission stages which follow the variance stage and are driven by the basic transmission input shaft. It comprises at least one, but often up to three further transmission stages. The overall transmission ratio of the small size transmission is the product of the transmission ratios of the variance stage and the further transmission stages. The housing, in which the basic transmission input shaft and at least one more transmission stage are located, forms the stationary part of the basic transmission, which is stationary in relation to the moving transmission parts, and to which the bearings of the gear shafts are fixed. The invention is not limited to a housing which is itself closed nor to a housing closed in conjunction with the motor.
According to the invention, the basic transmission input shaft comes in contact with the support element which is in a fixed position relative to the housing, when the variance gear is pressed to the preassembled basic transmission. The support element bears the pressure force necessary for pressing and distributes it over the housing. Thus, the power flow bypasses the rolling bearings of the basic transmission input shaft, so that damage of these due to the pressing action is avoided. Thus, it is possible to press the variance gear onto the readily assembled basic transmission with an axial force which is much higher than the axial load capacity of the rolling bearings in which the basic transmission input shaft is rotated. For the production of the transmissions, this has the essential advantage that a relatively small number of different basic transmissions can be pre-produced and held in stock, whereas the desired transmission ratios demanded by a customer can be realized simply and in short term, by pressing a variance gear providing (in conjunction with the appropriate pinion) the desired transmission ratio onto an appropriate basic transmission. This makes it possible to accomplish quickly and economically individual customer requirements, with limited storekeeping, for a vast range of transmission ratios.
It is not necessary to distribute the entire pressure force over the housing. Rather it is sufficient that this force is essentially taken over by the support element in the meaning of xe2x80x9cessentiallyxe2x80x9d, as to make sure that the rolling bearings of the basic transmission input shaft are not damaged during the pressing action. Preferably, the rolling bearings are ball bearings. The invention, however, also relates to transmissions with other types of rolling bearings, in particular roller bearings of different constructional forms. Subsequently, without limiting the universality, ball bearings are referred to exemplarily.
The procedure for the production of a small size transmission comprises a preliminary assembling step, in which the basic transmission is completely assembled, and a final assembling step, in which a variance gear, chosen according to the requirements, is pressed onto the basic transmission input shaft. This causes a reduction of the production costs as well as the storekeeping costs as compared to the earlier known manufacturing process, in which the variance gear is pressed onto the basic transmission input shaft before assembling the entire transmission. If in the latter case a short delivery time is required, the entire multitude of possible small size transmissions must be preproduced and kept in stock. The number of these is the number of different basic transmissions multiplied by the number of different variance gears, and is much higher than the total of basic transmissions and variance gears which have to be pre-produced and to be kept in stock according to the invention.
The pressure force, necessary for pressing-on the variance gear, can be reduced by preheating the gear, as generally known. However, this is only possible to a limited extent, depending on the used material. For example, parts hardened by case hardening can only be heated up to a maximum of 200xc2x0 C.
The invention is of particular importance for small size transmissions with a relatively small diameter of the basic transmission input shaft. In particular, it can be used for cases in which the diameter of the shaft, in the range of its bearings, is up to about 25 mm, preferably up to about 20 mm, and particularly preferred up to about 15 mm.
Small size transmissions with small shaft diameters have a small contact surface between the shaft and the variance gear and thus require a tight force fit in order to carry the required torque without slipping. Therefore, the force required to press the variance gear onto the shaft, must be relatively high. On the other hand, the haft bearings of small size transmissions are relatively weak and can only bear small axial loads. The invention solves this problem. For example, a pressure force of about 500 N was used for the pressing-on action in the case of a transmission, the ball bearings of which were able to bear a maximum axial force of 80 N.
Due to the design of the invention the basic transmission input shaft contacts, during the operation of the transmission, the support element which is fixed relative to the housing. Therefore a negative influence on the operation (e.g. an inadmissible increase of the operating temperature or a power loss) had to be expected due to increased friction and the resulting abrasion. Surprisingly, tests have shown that this does not occur. Preferably, the support element fixed relative to the housing is designed in such a manner that it is worn off in operation due to the contact with the basic transmission input shaft. Tests have shown that this xe2x80x9crunning-in processxe2x80x9d, which does not take place in the production plant, but in the normal operation of the transmission, does not affect the operational behavior to any significant extent. Preferably, the transmission is lubricated with an oil charge which absorbs the abraded particles as a dispersion.
The invention also comprises a series with a plurality of such small size transmissions. Such a series comprises small size transmissions of one or a plurality of constructional sizes, whereas a constructional size is defined by a certain center distance between the drive shaft and the basic transmission input shaft, when the small size transmission is connected to the drive. In order to provide a plurality of desired total transmission ratios for small size transmissions of the same constructional size, the invention starts from a common basic transmission. The desired overall transmission is then achieved by pressing-on in each case a suitable different variance gear for meshing with a respectively fitting drive pinion. A comprehensive series is formed by combining different variance gears with a number of different basic transmissions.
Preferably, the basic transmission input shaft is movable in relation to the housing within a given tolerance range s in axial direction. The tolerance range s of the basic transmission input shaft is limited by two positions, namely a first limiting position towards the input side and a second limiting position towards the opposite side. Furthermore reference is made to the distance between the basic transmission input shaft and the support. This is the distance between those parts of the basic transmission input shaft and the support which contact each other during the pressing-on of the variance gear onto the shaft. This distance varies within the tolerance range of the basic transmission input shaft. In the position of the basic transmission input shaft limiting the tolerance range in input direction this distance is denominated d. Preferably, this distance d is smaller than the tolerance range s. If this dimensioning rule is observed the basic transmission input shaft contacts, when it is shifted during the pressing-on of the variance gear onto the shaft in opposite direction to the input direction, the support before it reaches the opposite tolerance-limited position, in which one or both ball bearings of the basic transmission input shaft would be loaded. In this manner the pressing forces are transferred to the housing, whereas the ball bearings of the basic transmission input shaft are essentially completely bypassed.